1. Field of the Invention
The present invention relates to a chip antenna and to radio equipment including such a chip antenna. More particularly, this invention relates to a small-sized and broad-bandwidth chip antenna and the radio equipment including such a chip antenna.
2. Description of the Related Art
Up to now, in radio equipment such as a portable telephone terminal, a pager, etc., there have been used a wire antenna represented by a monopole antenna. When the radio equipment is made small-sized, the antenna is required to be of small size. However, in the case of a monopole antenna, as the length of a radiation conductor becomes .lambda./4 (.lambda.: wavelength of the resonance frequency), for example, about 4 cm in the case of an antenna having 1.9 GHz as its resonance frequency, the antenna itself comes to be too large, which means a problem because the need for small size cannot be satisfied.
To overcome the above problem, the present applicant has proposed a chip antenna as shown in FIG. 12 herein and in Japanese Unexamined Patent Publication No. 8-316725. The chip antenna 50 comprises a basic body 51 of a rectangular solid made up of dielectric ceramics containing barium oxide, aluminum oxide, and silica as its main components, a conductor 52 spirally arranged inside the basic body 51, and a feeding terminal 53 for applying a voltage to the conductor 52 formed on the surface of the basic body 51. One end of the conductor 52 is led out to the surface of the basic body 51 and connected to a feeding terminal 53. Further, the other end of the conductor 52 is made a free end 54 inside the basic body 51.
In the above construction, a small-sized chip antenna 50 has been realized by means of the spirally disposed conductor 52.
Generally, the resonance frequency f and bandwidth BW of a chip antenna are expressed as in the following equations: EQU f=1/(2.pi..multidot.(L.multidot.C).sup.1/2) (1) EQU BW=k.multidot.(C/L).sup.1/2 (2)
where L is the inductance of the conductor, C is the capacitance produced between the conductor and ground, and k is a constant.
FIG. 13 shows the frequency characteristic of the reflection loss of the chip antenna 50 of FIG. 12. From this drawing, it is understood that the bandwidth of a chip antenna 50 giving two or more of VSWR (voltage standing wave ratio) is about 225 MHz around the center frequency of 1.95 GHz.
However, in the case of the above-mentioned chip antenna, as the conductor is spirally arranged in order to make the chip antenna small-sized, the inductance L of the conductor becomes large. As a result, as clearly understood from Equation (2) there is a problem that as the inductance L of the conductor increases the bandwidth BW is narrowed.